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Who Is the Most Slothful Sloth?

This guy.

Sloths have a reputation for being…slothful. These tree-dwellers expend minimal energy throughout the day, moving so slowly that they can support entire ecosystems of moths, beetles, and algae on their fur. But according to a study published last month in the journal American Naturalist, some sloths are more slothful than others.

A team of researchers from the University of Wisconsin-Madison visited tropical forests, cacao agroforests, and monocultures of banana and pineapple in northeastern Costa Rica — home to two-toed sloths and three-toed sloths.

These two groups of sloths are separated by tens of millions of years of evolution. In that time, the families have adopted widely different lifestyles. While three-toed sloths occupy small home ranges and selectively eat leaves from only a few types of trees, two-toed sloths possess larger home ranges and enjoy a more diverse diet.

The researchers were interested in whether these sloths might also be paying different metabolic prices for their distinct lifestyles. To quantify metabolic rates, the team caught sloths, injected them with water containing unique forms of oxygen and hydrogen atoms (isotopes), and released them back into their habitat.

After a few days, the team re-captured those sloths and collected blood samples to measure how much of each isotope had been washed out of the body, which is a good way estimate of how fast their metabolisms were working over that period.

Though both sloths used very little energy, the metabolic rate of the three-toed sloth was found to be 31 percent lower than that of the two-toed sloth. Even more impressive, when the researchers compared metabolic rates across a wider set of species, they found that the three-toed sloth had the lowest energy expenditure recorded for any free-living mammal in the world.

“There seems to be kind of a cool combination of behavior and physiological characteristics that lead to these tremendous cost savings for three-toed sloths,” lead author Jonathan Pauli told ScienceNews. “They don’t do a lot of movement, whereas two-toed sloths are much more mobile.”

But there is more to energy saving than simply keeping still.

“Three-toed sloths have the capacity to fluctuate their body temperature,” he says. Most mammals, including humans, need to amp up their metabolism to maintain a specific core temperature. Three-toed sloths, on the other hand, allow their body temperature to rise and fall with the ambient temperature. “Those are big cost savings to let your body change with your surroundings.”